DHAKA: Families and rights groups Wednesday expressed fears for two survivors of a deadly siege at a Bangladesh cafe who are missing after being grilled by police over the attack.

Amnesty International (AI) has asked the authorities to establish "the fate and whereabouts" of Hasnat Karim who survived the attack and has been missing since being taken in for questioning 11 days ago.

Family members of Tahmid Khan also told AFP that they were in the dark about the 22-year-old Toronto University student's whereabouts after he was taken into custody as part of a police probe into the attack.

Suspected militants killed 20 diners and two police officers when they raided the upscale Holey Artisan restaurant on the night of July 1.

Army commandos stormed the cafe the next morning, killing all five attackers and rescuing 13 people, including Karim and Khan.

Police have said both were initially interrogated as they tried to piece together what had happened during the siege.

But police now say that the pair are no longer under their custody.

"We've questioned them immediately after they were rescued. But they are no longer in police custody," Dhaka police spokesman Masudur Rahman told AFP on Wednesday.

A military spokesman also told AFP that the two were not in their custody.

Fears for the pair's safety have been compounded after a 18-year-old injured survivor, who was rescued during the siege and was described as suspect, died in hospital after claims by his father that he was tortured by security forces.

Relatives of Karim and Khan insist both men have no connection to the attack which was claimed by the militant Islamic State (IS) group.

Karim's wife Sharmina Parveen, who was also held hostage along their two children, said she was afraid for his well-being.

"My husband is innocent. He has suffered enough. Please let him come home to his family," she said in a statement to a local rights group.

Reports in local media said both were being investigated for suspicious activity during the siege. They said Khan was seen holding a firearm and Karim strolling with the attackers on the roof.

"We understand it's a national security issue... But at least they should say where he is and allow our parents to see him," Khan's brother Talha Khan told AFP by phone from Toronto.

"They [Hasnat Karim's family] have already suffered a traumatic episode, and his enforced disappearance prolongs their ordeal," said Champa Patel, AI's South Asia Director.